The present invention relates to gas treatment of web material, and more particularly to an apparatus and process useful in connection with the manufacture of fire retardant textile fabric. The invention is particularly intended for use in connection with the THPOH--NH.sub.3 system of fire-retardant fabric treatment. Tetrakis (hydroxymethyl) phosphonium hydroxide is herein designated THPOH and is the product obtained by adding a base, e.g., NaOH, to tetrakis (hydroxymethyl) phosphonium chloride.
In the manufacture of textile fabrics, the art of rendering various types of materials flame resistant has taken on increasing importance, particularly in connection with fabrics used in the manufacture of wearing apparel. A known system for rendering certain types of textile fabrics flame resistant is the THPOH-NH.sub.3 chemical system which is particularly suitable for fibrous materials such as cotton knit and flannelette fabrics, which are among the most common fabrics used for nightwear and the like. Interest in the THPOH--NH.sub.3 system has grown steadily since it was first introduced because it can be applied to lightweight fabrics to render them flame resistant without adversely affecting other properties of the material. Generally, fabrics treated by this process have an improved strength retention, good hand and they may be processed in a manner which maintains their durability to repeated laundering. Thus, the THPOH-NH.sub.3 system has taken on increasing importance in connection with textile manufacture and, accordingly, greater need arises for apparatus and methods whereby this system may be more advantageously utilized in high-volume production facilities.
The THPOH--NH.sub.3 chemical system is usually practised by first treating the textile fabric with the THPOH, which becomes impregnated into the fabric. Subsequently, the fabric is exposed to ammonia gas which reacts with the THPOH to form a polymer in the fabric thereby enhancing the flame resistance of the fabric. After exposure to ammonia gas, the fabric is usually caused to undergo subsequent treating procedures, such as an oxidation step. But for the purposes of the present invention, it is only significant to consider the fact that fabric impregnated with the THPOH must be subsequently treated with ammonia gas and that during this procedure certain very significant problems and difficulties will arise which may seriously impede the effectiveness of the treatment. Thus, for the purposes of the present disclosure it is not deemed necessary to provide extensive details of the chemical aspects of the THPOH-NH.sub.3 system inasmuch as this information may be obtained from prior art publications such as U.S. Pat. No. 3,607,356, incorporated herein by reference. From these publications it will be seen that various approaches may be utilized with regard to the chemical aspects of the process and that from the point of view of the apparatus which is required to effect the process, several difficult problems will be encountered.
One of the more important problems which is encountered relates to the handling of the ammonia gas, which is a highly noxious and toxic gas. Thus, in the fabric treatment process, great care must be taken to avoid the escape of undue amounts of ammonia gas into the environment where the process is being practiced. Since exposure to ammonia gas could have a damaging effect upon workers and other personnel occupying areas in proximity to the treatment equipment, the process whereby the THPOH impregnated fabric is exposed to ammonia gas must be carried out within a sealed chamber.
Prior art techniques for practising this aspect of the treatment process have involved passage of the impregnated fabric through a sealed tank containing an atmosphere of ammonia gas. The fabric is wound upon rollers located within the sealed tank and ammonia gas is caused to flow through the tank in order to expose the fabric thereto. These prior art approaches have generally involved merely filling the internal volume of the tank with ammonia gas and moving the fabric through the tank to effect the needed exposure. Thus, the prior art has almost exclusively practised what is commonly called a "surface adhesion" technique, wherein the impregnated fabric is simply exposed within an atmosphere permeated with ammonia gas. Although it is possible with such prior art techniques to produce a fabric having an acceptable degree of flame resistance, it has been found that many serious drawbacks will arise.
For example, since it is required that the ammonia tank be maintained in a sealed condition it is not feasible to insert and remove fabric from the tank in finite batches. Thus, the fabric must be continuously moved through the tank and as a result there arises a need for an entrance seal and an exit seal through which the fabric may enter and leave the tank without permitting escape of ammonia gas. An added difficulty is the fact that the tank must be capable of handling fabric of different widths. Accordingly, the problem of permitting fabric to move through the tank at speeds sufficient for mass production volume while also maintaining an adequate seal for fabrics of different widths will present severe obstacles to the adoption of the THPOH-NH.sub.3 system as a practical approach to the manufacture of flame-retardant fabrics.
Other problems will be encountered which relate to the fact that during the process of exposing the fabric to the ammonia gas, water is formed as a product of the reaction between NH.sub.3 and THPOH. If this water is allowed to accumulate within the tank to any significant degree, it will adversely affect the treatment reaction causing fabric to be produced which does not have the required degree of fire retardance. Since the moisture which is produced is a product of the reaction between the ammonia gas and the THPOH, control of moisture content within the treatment chamber will be dependent upon the quantity of ammonia gas which is introduced and the rate of flow with which the reaction products are removed from the tank. With presently known techniques, it has been found virtually impossible to control these important parameters of the treatment process in a manner which avoids the adverse effects of the moisture while producing a satisfactory end product.
Since the prior art relies upon mere exposure of the THPOH-impregnated fabric to ammonia gas, such processes require high level quantities of ammonia within the treatment chamber in order to produce a useable fabric. Furthermore, the time during which the fabric must be exposed to the ammonia is relatively long. Accordingly, reaction products cannot be removed from the tank at elevated flow rates. Otherwise, insufficient fabric exposure to the ammonia gas will result, and an end product which is not sufficiently fire resistant will be produced. However, failure to remove the reaction products at a suitable rate will cause moisture to accumulate at the reaction site thereby inhibiting the THPOH-NH.sub.3 reaction and rendering the fabric insufficiently flame resistant.
Therefore, if the THPOH--NH.sub.3 system of manufacturing flame resistant fabrics is to achieve practical utilization, it is necessary that there be provided manufacturing apparatus and techniques which will overcome the previously mentioned obstacles.